Sensors and sensor networks are extensively used in different applications and in particular in applications of image or scene interpretation. In these applications a first objective is identifying temporal changes to the illumination level at any point in the image. In a regular camera sensor such as a CMOS or a CCD sensor, photons that impinge on the active pixel area are transformed into electrons that are retained in the pixel by its design. Subsequently, the captured electrons are moved out of the pixel area, their movement or electric current are transformed from analog to digital values, and the digital values are output to a receiving module for further processing. Movement of electric charges is by definition consumption of electrical energy. In existing cameras, the sensor alone consumes at least 30 mW of electric power. Subsequent processing by a processor requires substantially more power.
In some applications such as vehicle collision prevention, machining or semiconductor processing equipment, visual sensors can be mounted on the equipment itself to monitor operation of the equipment. In such applications sensors receive sufficient electric power supply from the adjacent equipment enabling their operation. Other applications such as surveillance, people tracking and access control require event monitoring across large areas over long periods. In such applications a number of sensors are distributed across the surveilled area, power being supplied by expensive infrastructure. In addition to power supply, communication means are required for transmission of captured images or image analysis results. In most of these applications, the time between events of interest (EOI, or alerts) is large with respect to the duration of the event itself, resulting in a very small operating duty-cycle. This means that a standard camera-processor setup must work continuously, capturing images, sending them to the visual processor, which processes them and most of the times discovers that there is no alert to send or decision to take. Nevertheless, continuous scene monitoring consumes significant electric power, and requires expensive and complex infrastructure, limiting its efficacy and applicability.
Electrical power supply as well as communications channel to a distributed sensor network over a large area is complex and expensive. Current sensors are often bulky, expensive, and sometimes limited to image recording or capture. Surveillance and similar tasks execution requires continuous sensors and in particular visual sensors operation. Such operation consumes hundreds or thousands of milliwatts and accordingly the sensors depend heavily on external power supply, limiting their usage. Sensors being capable of interpreting the visual world while consuming only a few milliwatts could be densely deployed to be employed in applications such as smart buildings, assisted living, safe cities, long perimeters and many more.